In many communication systems retransmission protocols are used in order to handle erroneous transmissions, i.e. transmission attempts where the receiving end is not able to decode the content. When a transmission attempt has failed the receiving end informs the transmitting end through a feedback channel. One type of feedback information consists of an indication on whether the transmission attempt was successful or not. For a successful transmission the receiver acknowledges the transmission (ACK) while a negative acknowledge (NACK) follows an unsuccessful transmission attempt. The transmitting end uses the feedback information to determine whether a retransmission and/or additional redundancy is needed or not.
In some cases, for example in the uplink of Code Division Multiple Access (CDMA) systems in soft handover, several receiving units (Node Bs) may be involved in the reception of a transmission. The reception attempts from the receiving Node Bs are combined in centralized units, such as the Radio Network Controller (RNC) in the Wideband Code Division Multiple Access (WCDMA) specifications, and a retransmission is only needed in case none of the receiving units are successful in the reception.
In current proposals for enhancing the CDMA standards (WCDMA and CDMA2000) it is proposed that the feedback information (ACK/NACK) is to be generated directly in the Node Bs to enable faster retransmission. Each receiving end (Node B) involved in the reception has a separate feedback channel to transmit an ACK or a NACK depending on the outcome of the decoding attempt. The transmitting end (user equipment) performs a retransmission only if a NACK is obtained from all receiving ends. If an ACK is received from any of the receiving units the transmission attempt is considered successful and no retransmission is performed. This means that although an individual receiving unit was not able to decode and responded with a NACK, the combined transmission outcome may have been successful.
In many cases the Hybrid Automatic Retransmission Request (HARQ) processes perform “soft combining”, i.e. the retransmitted signal is combined with the previous transmission(s) to increase the probability of successful reception. In these cases all the receiving units must know if a transmission is a retransmission, which should be combined with previous transmissions, or if it is an initial transmission corresponding to new data.
To inform all receiving units of the combined outcome the transmitting unit indicates, for each transmission, if it is a retransmission or a new transmission. This is, for example done by transmitting a Retransmission Sequence Number (RSN) in combination with each transmission. The RSN is used to convey to all receiving units the transmission number for each transmission. If the RSN indicates that the current transmission is a new transmission the transmission is not combined with previous transmissions. If the RSN indicates to all receiving units that the current transmission is a retransmission the transmission is a repetition of previously transmitted data and/or additional redundancy (check bits) related to previously transmitted data which should be combined with previous transmissions.
In previous releases of the standards the combining of the uplink transmissions is done in the RNC. If at least one Node B is able to decode a transmission, it is acknowledged (ACK).
In case of an enhanced uplink, which is currently being standardized within the Third Generation Partnership Project (3GPP) for WCDMA, each Node B involved in the decoding transmits a separate ACK/NACK feedback. The user equipment considers the feedback information from e.g. two Node Bs. As an example, a first Node B is able to receive the transmission and sends an ACK to the user equipment on the feedback channel, while a second Node B was not successful and transmits a NACK. If any of the Node Bs send an ACK there is no need for a retransmission so the user equipment does not retransmit. Observe that the second Node B does not know if the first Node B was successful or not and must be informed by the user equipment. Since the transmission was successful the user equipment increments the RSN and transmits the same transmission in combination with the next transmission. The second Node B is thereby informed that the combined transmission was successful and that the new transmission is not a retransmission of previous data.
For the retransmission protocol to be stable the feedback channel that carries the ACK/NACK information must be reliable, errors in the signalling will cause protocol errors and increased delays and degraded throughput. In order to achieve reliable feedback information the power needed may be substantial and this will affect the performance of the downlink communication as there will be less power available. The problem is especially serious in situations where several receivers are involved in the reception (soft handover). In these cases additional power must be added to the feedback channel to guarantee reliable reception. The existing solution is to use additional power for these situations regardless if it is needed or not leading to over-provisioning and excessive power consumption. Less power is then available for downlink oriented data leading to decreased downlink performance.
There is, therefore, a need for an improved method and arrangement for efficiently allocating the available radio resources in a mobile communication network.